'Sex, lasers and male competition' — '80s action flick or new fly study?

UC researchers find unfit flies lose genetic race even after mating

Scientists have accepted natural selection as a driver of evolution for more than 160 years, thanks to Charles Darwin.

But University of Cincinnati biologist Michal Polak says Darwin’s book “The Descent of Man” only tells part of the story. Sometimes when the victor vanquishes his sexual rival, the quest to pass genes to the next generation is just beginning.

According to a new UC study published in the journal Current Biology, male fruit flies with the most impressive sexual ornamentation also have super sperm that can outcompete that of rivals in the post-mating fertilization game.

The study demonstrates UC's commitment to research as described in its strategic direction called Next Lives Here.

UC biologist Michal Polak, professor of biology, has discovered a new species of fruit fly. Shown here in his lab.

UC biology professor Michal Polak found a correlation between fruit flies with the most impressive sexual ornamentation and their competitive fertilization success. Fruit flies have a specialized comb on their front legs they use to attract mates. Photo/Andrew Higley/UC Creative + Brand

UC studied Drosophila bipectinata, a tiny red-eyed fruit fly from the South Pacific. The male’s forelegs have a distinctive “sex comb,” dark bristles that female fruit flies find appealing — like the colorful train of a male peacock. Scientists previously found that female flies prefer males with more robust sex combs, which the males use to grasp the female’s abdomen before mating.

An illustration of two mating fruit flies with an inset of the male's sex comb on it foreleg.

UC researchers studied the fruit fly Drosophilia bipectinata. The male uses a distinctive sex comb to attract a mate. Graphic/Michal Polak

UC researchers in Polak Lab found a strong link between the most impressive sex combs and that male’s competitive success at passing on his genes even after a female fly has mated with other flies. And this competitive edge persisted even after the male’s sex comb was surgically removed with a high-precision laser in UC experiments.

“This is the first robust demonstration of a genetic link between a traditionally Darwinian trait and success in postcopulatory sexual competition,” Polak said. “That’s the surprising link: precopulatory and postcopulatory fitness.”

In his groundbreaking 1859 book “On the Origin of Species,” Darwin framed the idea of natural selection by describing how the “fittest” animals pass on their genes to the next generation. This fitness is manifested in having the largest antlers, the most vibrant colors or the vigor to defend a territory.

But Darwin’s theory was incomplete, Polak said, because it failed to recognize that sexual selection continues during and after mating. Female fruit flies are promiscuous, often choosing multiple mates. Fruit flies are hardly alone in that regard, Polak said.

“Promiscuity is much more common across animal species than once was thought,” Polak said. 

Scientists living in prim and proper Victorian England did not give enough consideration to the microscopic race to fertilize that begins after mating among multiple successful suitors.

“You have to consider the social context in which Darwin was living,” Polak said.

There aren’t many studies more interesting than this one – sex, lasers, and male competition, which could describe most 1980s action movies.

Joshua Benoit, UC biologist

What females gain from mating with multiple suitors is not always clear, Polak said. But when they do, postcopulatory sexual selection provides a competitive edge.

“It’s evident even in primates. Female chimpanzees and bonobos are promiscuous, so the males have large testes that produce big volumes of sperm,” Polak said.

“And you have species like gorillas where females are not promiscuous. Silverback males enforce monogamy. And lo and behold, their testes are much smaller relative to body size compared to chimps.”

And if you’re wondering, the relative size of human testes falls somewhere between gorillas and chimps, Polak said.

UC biologist Michal Polak, professor of biology, has discovered a new species of fruit fly. Shown here in his lab.

UC biologist Michal Polak displays mounted specimens of flies from around the world in his lab. Photo/Andrew Polak/UC Creative + Brand

Polak, a professor of biology in UC’s College of Arts and Sciences, decided to study this species of fruit fly after encountering it while conducting fieldwork in Queensland, Australia.

“I was watching these flies mate on a fruit and looked under the microscope and saw these beautiful sex combs. I thought it would make a good model system to study,” Polak said.

“Sexual selection picks up on these traits and they become really exaggerated,” he said.

For their study, UC biologists artificially selected males with the largest and smallest sex combs in 11 successive generations of fruit flies to create high and low genetic lines.

Kassie Hooker from 2012 to 2015 worked in Polak’s lab as an undergraduate biology student, undertaking the arduous task of categorizing generations of male fruit flies based on the size of the sex combs on their legs. By counting the teeth in each comb, she separated the males with the largest and smallest sex combs to create distinct genetic lines.

“Many researchers use these physical characteristics as an indicator of genetic quality and reproductive fitness in male Drosophila,” Hooker said.

UC biologist Michal Polak, professor of biology, has discovered a new species of fruit fly. Here, his research assistant, graduate student Cassie, works in the lab.

UC graduate student Kassie Hooker worked on the study on fruit flies as an undergraduate biology student. Photo/Andrew Higley/UC Creative + Brand

To show that the male fruit fly’s sex comb doesn’t provide any reproductive benefit in mating, researchers used ultraprecise lasers to trim the sex comb in the high line males to mimic those found in the low line males. But these postsurgical males continued to fertilize more eggs even when females mated with lower-line males first.

Hooker is pursuing her master’s degree in clinical and translational research in the UC College of Medicine Department of Surgery, where she works as a research coordinator.

“I credit my involvement as a member of the Polak Lab as the experience that sparked my interest in science and research as a career,” she said. “I can confidently say my experience in undergraduate research at UC has given me numerous opportunities and continues to help me grow in my career and in my life.”

The research was supported by the National Science Foundation.

UC assistant professor Joshua Benoit, a study co-author, analyzed the RNA of the flies and identified seminal fluid genes that may be responsible for giving high-line males a fertilization advantage.

“There aren’t many studies more interesting than this one,” Benoit said. “Sex, lasers, and male competition, which could describe most 1980s action movies.”

Darwin proposed the theory of sexual selection to account for the evolution of male weaponry and extravagant ornamental displays, Polak said. But UC’s study found a far more complex and interesting battle among the sexes.

“We established a link between Darwinian traits and the postcopulatory arena, which Darwin didn’t recognize was important in evolution,” Polak said.

Featured image at top: Kassie Hooker, left, a UC master's student, UC assistant professor Joshua Benoit and UC biology professor Michal Polak collaborated on a study that linked sexual ornaments called sex combs with a male's postcopulatory reproductive success. Photo/Andrew Higley/UC Creative + Brand 

UC biologist Michal Polak, professor of biology, has discovered a new species of fruit fly. Shown here in his lab.

UC biology professor Michal Polak holds up an poster in his lab celebrating his discovery of a new species of fruit fly to be described in a future paper. Photo/Andrew Higley/UC Creative + Brand

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